Abstract : During the 1974-1976 report period our principal efforts were directed in the following areas: (1) determining the applicability of an ultrahigh strength graphite fiber to composite pressure vessels; (2) defining the fatigue performance of thin titanium lined, high strength graphite/epoxy pressure vessels; (3) selecting epoxy resin systems suitable for filament winding; (4) studying the fatigue life potential of Kevlar 49/epoxy pressure vessels; and (5) developing polymer liners for composite pressure vessels. The fiber performance and potential as a reinforcement for composites of the Thornel Special graphite fiber were evaluated by testing fiber/epoxy strands. Properties determined were fiber uniformity, strength and modulus at room and liquid nitrogen temperatures, stress strain behavior, effect of strain rate on fiber strength, and acoustic emission during tensile loading to failure. The Thornel Special graphite fiber was found to have a 357O-MPa failure stress, a 1.7% failure strain, a 2O6-GPa modulus, and a 1.77-Mg/m3 density at 230C. Low temperature and various strain rates appear to have no significant effect on fiber tensile properties.